1. Field of the Invention
This invention relates to a rubber composition suitable for use in tires, particularly bead filler, and more particularly to a rubber composition developing good rigidity and durability as a bead portion of the tire by compounding a novel modified novolak resin.
2. Related Art Statement
Heretofore, there have been made various investigations on the bead portion structure of the radial tire in order to develop the rigidity and durability required for the tire.
For instance, there is attempted the arrangement of a bead reinforcing layer in the bead portion for improving the running performances, durability and the like of the tire.
On the other hand, the arrangement of super-hard rubber in the bead portion for improving the running performances and the like are well-known from Japanese Utility Model Publication No. 47-16,084, French Patent No. 1,260,138, U.S. Pat. No. 4,067,373 and so on.
However, these conventional techniques do not provide satisfactory solution as mentioned later. In Japanese Patent Application Publication No. 57-30,856, therefore, there is proposed a rubber composition for bead filler obtained by compounding novolak type phenolic resin and hexamethylene tetramine (hereinafter referred to as hexamine) or hexamethoxymethyl melamine with natural rubber, polybutadiene rubber or the like together with carbon black. Moreover, Japanese Patent Application Publication No. 57-30,856 discloses the addition of novolak type modified phenolic resin, which is a resin modified with an oil such as rosin oil, tall oil, cashew nut oil, linseed oil, olein oil or the like; an aromatic hydrocarbon such as xylene or the like; or a rubber such as nitrile rubber or the like. This modified novolak resin is entirely different from the modified novolak resin defined in the invention as mentioned later.
In the aforementioned method of arranging the bead reinforcing layer in the bead portion, the number of tire building steps becomes larger, and the tire productivity is considerably poor.
Further, in the method of arranging the superhard rubber in the bead portion, it is hardly considered to give the durability required as the tire while sufficiently developing performances as a bead filler rubber under complicated inputs during the running of the tire.
In the technique disclosed in Japanese Patent Application Publication No. 57-30,856 for solving the above problems, the novolak resin is cured in rubber with a curing agent such as hexamine, hexamethoxymethyl melamine or the like, so that the curing efficiency is poor as compared with the ordinary resin molding based on the mixing of only resin and curing agent and consequently a great amount of unreacted resin remains in rubber. Therefore, in order to provide a hardness desired as a bead filler rubber, it is necessary to increase the amount of the resin compounded or to increase the amount of the curing agent.
However, when the amount of the resin to be compounded is increased, the amount of unreacted resin becomes naturally large. As a result, the unreacted resin degrades the mechanical properties of the resulting rubber composition, particularly fatigue life and creep properties as a mere foreign matter and also provides a large heat build-up to shorten the fracture life of the tire. On the other hand, when the amount of the curing agent is increased, in case of hexamine, the strength of polyester fiber cord usually and widely used as a reinforcing cord for a carcass ply adjoining to the bead filler is lowered due to amine degradation during the tire vulcanization or during the running of the tire, which particularly comes into problem when the tire is vulcanized at high temperature. In case of using hexamethoxymethyl melamine, there is not caused the lowering of strength of the polyester fiber cord, but there are still the same problems as in the presence of the unreacted resin because the curing efficiency is much lower as compared with the hexamine.